Centre d’Élaboration de Matériaux et d’Etudes Structurales (UPR 8011)


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INCA INfluence of local Chemistry on the mechanical behavior of metallic Alloys

 

Coordinators : F. Pettinari-Sturmel (PPM), B. Warot-Fonrose (I3EM).

Participants :

M. Hantcherli, J. Douin, J-P. Monchoux, A. Couret, M. Legros (PPM)
M. Brunet, P. Sciau (M3)
C. Marcelot, : S. Joulié, N. Ratel (Plate-form characterization)

 

Context

Metallic alloys for structural application have a complex microstructure consisting in a solid solution containing major elements (Ni, Ti, Al) and a few percent of solute elements. The mechanical strength of these alloys originates in the solid solution and / or in microstructural elements associated to solutes (such as precipitates).

Numerous investigations of solid-solution effect in nickel-based single crystal alloys have shown that rhenium, tungsten and molybdenum are good candidates for improving high-temperature strength, with the most important effect being attributed to rhenium. More recently, studies paerformed in TiAl alloys have also revealed an improvement in mechanical properties through the addition of a few percent tungsten.

Even if solid solution effects, particularly those caused by the addition of a few percent rhenium have been extensively studied in nickel solid solutions [Matsuo et al. 1987, Nathal et al. 1989, Shinoda et al. 1987, Clement et al. 1996, Pettinari et al. 2001], the solid-solution hardening mechanism, which remains very complex, particularly at high temperature, remains unclear.

From a fundamental point of view, the relevant physical parameters at the origin of the beneficial effects of rhenium and / or tungsten on the mobile dislocations during the deformation, are still to be identified to specify their hardening effects at the nanoscopic scale.

In the case of aluminum alloys, the structural hardening due to a fine precipitation and the evolution of this precipitation during aging, are complex. The origin of rapid hardening in Al-Cu-Mg alloys has, for example, been controversial until very recently [Wang et al. 2007, Marceau et al. 2010, Styles et al. 2012] : the precipitation sequence of the S phases with the formation of Cu-Mg clusters in the very first step of aging, their role in the precipitation sequence, the crystallographic structure of the S ’and S’ phases have been identified combining DSC, atomic probe and high resolution TEM (associated with electron diffraction and EDX or EELS spectroscopies). However, the understanding of the long term aging of these alloys remains a current issue : the study after long annealing at low temperatures (between 70 and 100 ° C.) [Deschamps et al. 2017] or the direct study of aged alloys on end-of-life aircraft [Salimon et al 2010, Cochard et al. 2017] make it possible to tackle the problem. Slow phenomena modeling involving fine local variations of chemistry in and around precipitates should be continued.

To conclude, although some of these local chemical effects have been already studied and are known, they are still partially misunderstood and have to be analyzed.

 

Objectives

This “transverse research operation” is aimed to analyse of the influence of local chemistry on the physical mechanisms, which control the high-temperature mechanical behavior in different class of metallic alloys : Nickel base superalloys, aluminum alloys, Titanium aluminides (TiAl). This project is based on TEM advanced quantitative techniques available at the CEMES and UMS Castaing (TEM probe corrected for chemical analysis using spectroscopy, in situ TEM for deformation micromechanisms, Spherical aberration corrected TEM for structural analyzes). Some supplementary experiments using X-ray or neutron diffraction could be performed, to identify and quantify the relevant physical parameters which are influenced by "local chemistry" and which control the elementary deformation mechanisms. The effects of the solutes will be studied. It can be direct (ie solid solution effects encountered by the dislocations in monocrystalline superalloys and in TiAl) or indirect (ie modification of order and stacking energies that control dislocation mobilities in connection with the chemistry of the matrix and / or precipitates in the case of polycrystalline superalloys and aluminum alloys).

The final objective is to participate to the development of a mechanical model integrating internal microstructural variables related to local chemistry, which are relevant for the mechanical properties.

This scientific project is based on various research topics (elementary mechanisms of plasticity, local stoichiometry, dislocation interactions / microstructural elements) and concerns different classes of alloys. This “transverse operation” mobilizes thus a large community of researchers from different research groups of the CEMES.

 

Project timelines :

Here below, a list of the objectives to 2-3 years (corresponding to submitted projects and / or thesis in progress) :

  • Characterization of the local chemical compositions using EELS / EDX spectroscopy, implementation of new protocols / methodologies and experiment using RX or neutron diffraction if necessary.
  • Identification of the deformation mechanisms as a function of solute content / precipitation.
  • Study of dislocation-solute / precipitate interactions.

 

Experimental means used

  • CEMES and UMS Castaing TEM equipments : corrected probe TEM for spectroscopy, TEM in situ for deformation mechanisms, spherical aberration corrected TEM for structural analyzes.
  • Preparation platform of the CEMES.
  • if necessary : Synchrotron diffraction or neutron scattering, and atom probe.

 

Financial supports

  • Projet IDEX COCAGNE (2016-2017)
  • Projet NEXT StelAir (2016-2018)